Confused a bit!

Yesterday I went in for a doctors appointment and the doctor brought his handheld ultrasound machine in and did a quick scan of the babies. They are sitting right next to each other now. When I was 7 weeks 1 day they were far apart. He said that they are in the same amniotic sac. So I go back on the 23rd to see if they share a placenta. What are the odds that they don't share one? There is also no membrane between them